Discrimination between hydrogen atom and proton abstraction in the quenching of n,π* singlet-excited states by protic solvents

The fluorescence quenching of the azoalkane 2,3-diazabicyclo[2.2.2]oct-2-ene by protic solvents (methanol, methanol-OD, water, deuterium oxide, acetic acid) has been examined. The pseudo-unimolecular quenching rate constants (k(q)) vary from 0.30-44x10(6) s(-1) and decrease upon deuteration of the solvent OH bonds, e.g., the isotope effect for methanol/methanol-OD is ca. 8.5. This demonstrates that the hydroxylic hydrogens are predominantly responsible for the fluorescence quenching. The activation parameters were determined for methanol and methanol-OD. The activation enthalpies are unexpectedly low (Delta H double dagger = 1.8 kcal mol(-1) for methanol) and increase upon deuteration (Delta H double dagger = 3.0 kcal mol(-1) for methanol-OD), while the activation entropies remain the same (Delta S double dagger ca. 17.5 cal K-1 mol(-1)). This provides evidence for a fully classical isotope effect related to differences in zero-point vibrational energies. Tunneling appears to play no significant role. The quenching rate constants display no trend with the acidity of the solvent (pK(a) values) but with the hemolytic bond dissociation energies of the OH bends. This suggests the involvement of a hydrogen atom rather than a proton transfer. All important aspects (activation enthalpies, isotope effects, etc.) of this novel quenching mechanism of protic solvents are reproduced by MCSCF quantum-chemical calculations with a complete active space of CAS (12,10) or CAS (8,7). Most importantly, the computed data indicate the occurrence of a conical intersection, i.e., a real surface crossing, which follows the transition;state and provides an efficient trigger for radiationless return to the ground-state energy surface (fluorescence quenching).